do i need to pipe radiator as two pipe , or can i increase single pipe supply/return to 1 1/4 and get heat?++ There is no reason a one-pipe radiator will work on a two-pipe system. A one-pipe system uses the one-pipe main both as supply and return. Taking a one-pipe radiator off a two-pipe supply main leaves no way to return condensate, so the radiator fills with condensate at the bottom and stops heating. You will need the return added to make the new radiator a two-pipe, piped with a supply off the 2-pipe supply main and a radiator return to the 2-pipe condensate return main.